1. Field of the Invention
The present invention relates to methods and apparatus for cancelling inter-symbol interference (ISI) in coded spread spectrum communication systems.
2. Discussion of the Related Art
In an exemplary wireless multiple-access system, a communication resource is divided into code-space subchannels, which are allocated to different users. A plurality of subchannel signals received by a wireless terminal (e.g., a subscriber unit or a base station) may correspond to different users and/or different subchannels allocated to a particular user.
If a single transmitter broadcasts different messages to different receivers, such as a base station in a wireless communication system broadcasting to a plurality of mobile terminals, the channel resource is subdivided in order to distinguish between messages intended for each mobile. Thus, each mobile terminal may decode messages in its allocated subchannel(s) from the superposition of received signals. Similarly, a base station typically separates received signals into subchannels in order to differentiate between users.
In a multipath environment, received signals are superpositions of time-delayed and complex-scaled versions of the transmitted signals. The multiplicity of time-delayed paths effectively causes the transmitted waveform to spread out in time en route to the receiver. Several types of interference can arise in multipath channels. Intra-channel interference occurs when multipath delay causes leakage between subchannels. For example, forward-link subchannels that are orthogonal at the transmitter may not be orthogonal at the receiver. Leakage due to multipath delay can also occur within a subchannel when multiple symbols are transmitted sequentially (such as in a multi-shot transmission), as the modulated waveforms for different symbol intervals overlap each other at the receiver. This leakage is known as inter-symbol interference (ISI), which is defined as spurious contributions to a received signal in a given symbol interval due to symbols transmitted during other symbol intervals. ISI can be especially significant when the delay spread of the fading is a substantial fraction of the duration of a spreading code. Inter-channel interference occurs when multiple base stations (or sectors or cells) are active so that the received signal includes unwanted contributions from arriving signals sent by other base stations.
Interference can degrade communications by causing a receiver to incorrectly decode received transmissions, thus increasing a receiver's error floor. Interference may also have other harmful effects on communications. For example, interference may diminish the capacity of a communication system, decrease the region of coverage, and/or decrease maximum data rates. For these reasons, a reduction in interference can improve reception of selected signals while addressing the aforementioned limitations due to interference.
There are many techniques for dealing with intra-channel interference and inter-channel interference (e.g., maximum likelihood and linear minimum mean-squared error), including the low-complexity iterative interference canceller (IIC) described in Provisional U.S. Pat. Appl. Ser. No. 60/736,204. Embodiments of the present invention may work in conjunction with various interference cancellers in order to cancel ISI, in addition to intra-channel and inter-channel interference.